Electrodeposition of chromium and alloys thereof



United States Patent Of we ELECTRQDEPUSITION F CHROMIUM AND ALLOYS THEREOF Cloyd A. Snavely, Charles L. Faust, and John E. Bride, Columbus, Ohio, assignors, by mesne assignments, to The Batteiie Development Corporation, Columbus,

Ohio, a corporation of Delaware No Drawing. Application February 12, 1951, Serial No. 210,624

9 Claims. (Cl. 204-43) This invention relates to chromium coatings. More particularly, it relates to a method or process ofelectrodepositing coatings containing chromium which will retain hot hardness, and to the bath used and the coating produced thereby.

Recently, the use of chromium plating has been extended to other fields than that of decorative plating. Chromium plated surfaces are very hard, have a high resistance to wear, corrosion and erosion, and have a low coeflicient of friction. In addition, these surfaces can be given. a high polish which is relatively permanent.

Heretofore, chromium plating has been commercially practicable only when produced by chromic acid baths. Baths of this type may be prepared by dissolving in water chromic acid anhydride (CrOs) and a catalyst, such as a sulfate, fluoride, fluorosilicate, or other material known in the art.

In spite of the wide success of chromium plating from such baths, there are several operating features for which some improvement is highly desirable. For example, the current efliciency and energy efficiency are very low when compared to other metal plating processes. Also, the throwing power, that is, the ability to plate in recessed areas, is poor. This necessitates the use of special anode fixtures. In addition, the heavy gas discharge at the electrodes causes a corrosive spray from the surface of the plating bath. While these limitations have not proved prohibitive, they have made the plating process difficult and have added materially to the cost. They have also presented a definite health hazard.

Present-day chromium plate, although widely used for its property of extreme hardness, still has definite limitations as to the maximum temperature at which its hardness will be retained. The advantageous properties of chromium plate would. be greatly enhanced if hardness could be retained at relatively high temperatures. A hot-hard chromium plate, that is, one having stable hardness when heated, would be of great commercial significance for materials exposed to high temperatures.

The chromic acid bath, commonly used at the present time, does possess several advantageous features in that composition of the bath is simple, and analysis and control are not complicated. Chromium is easily replaced in the bath by adding CrO3, from which source chromium metal is obtained at a low per pound cost. However, the fumes from the chromic acid bath may prove dangerous to personnel if adequate ventilation is not provided. In addition, when thick coatings of chromium are formed, the surface tends to crack.

It is believed that a plating solution based on a trivalent chromium salt could eliminate most, if not all, of the difficulties arising from the use of the chromic acid bath. Although many attempts have been made to plate from a trivalent chromium bath, none of them have resulted in a commercially suitable chromium plating process.

It has now been found that a suitable trivalent chromium salt bath can be used to electrodeposit chromium metal on other metals with an electrical efliciency of deposition much higher than has heretofore been known, and forming chromium plate whose properties are of commercial interest. By using the electrolytic solutions of the composition hereinafter described, many of the difliculties encountered in the conventional chromic acid process are eliminated. Moreover, the addition of a controlled amount of iron or other metals in a chromium plating. solution will result in the electrodeposition of a v 2,693,444 Patented Nov. 2, 1954 coating having suflicient hardness to be useful where hard chromium deposits are desired. This coating is not brittle and does not have a tendency to soften appreciably at elevated temperatures.

It is, therefore, an important object of this invention to provide an electrolytic method for depositing chromium containing coatings at a high current efficiency and high electrical energy efficiency.

Another object of this invention is to provide electrodeposited hard chromium containing coatings which substantially retain their as-deposited hardness when exposed to elevated temperatures.

A further object is to provide a method of rapidly depositing a hot-hard chromium containing coating.

Yet another object is to provide a thick chromium containing coating which is free from customary crack patterns.

Other objects and advantageous features of this invention will become apparent from the following description, examples and appended claims.

In general, this invention involves making an article to be plated the cathode in an electrolytic bath, said bath comprising a mixture of chromium ammonium sulfate, ferrous ammonium sulfate, magnesium sulfate, sodium sulfite and water. A suitable electric current is passed through this solution for a sufiicient length of time to deposit the coating to the desired thickness.

The following examples will serve to illustrate the invention with greater particularity:

Example I An aqueous bath of the following composition was prepared:

Ammonium hydroxide [28%-NH4OH] ml./l 6O Chromium ammonium sulfate [Cr2(SO4)3'(NI-I4)2SO4-24H2O] g./l 700.0 Ferrous ammonium sulfate FeSO i- (NH4)2SO4- 6H2O] g./l 13.5 Magnesium sulfate [MgSO4-7H2O] g./l 20.0 Ammonium sulfate [(NH4)2SO4] g./l 50.0 Sodium'sulfite [0.005 g./n'1l.-Na2SO3] ml./l 50 A steel panel was immersed in the bath as the cathode, while the anode was a copper rod plated with 0.015-inch of an alloy comprising per cent lead and 10 per cent tin. The bath was maintained at a temperature of F, having a pH of 1.7. The panel was plated for 60 minutes, using a potential of 7 volts and a current density of 400 amperes per square foot. The coating was found to cornpriseabout 94 per cent chromium, the balance iron, and was 0.0l0-inch in thickness.

The chromium-iron alloy coating was tested and found to have a hardness of from 600 to 700 Knoop, which it retained up to a temperature of 1110 F. When the coated article was heated to 1800 F., the hardness was still 400 Knoop. In comparison with these values, it should be pointed out that chromium plate electrodeposited from the chromic acid bath has a hardness of only 200 to 300 Knoop when heated to temperatures above 900 F.

Example II A similar deposit of about 94 per cent chromium and the balance iron was also obtained from thefollowmg aqueous solution, but using a low current density:

Ammonium hydroxide [28%-NH4OH] ml./L 20 Chromium ammonium sulfate The bath was maintained at a temperature of 118 to 120 F. with a pH of 1.8, using a voltage of 4 volts and a cathode current density of amperes per square foot. The anode, cathode and other operating conditions were similar to those of Example I.

Example III A bath was prepared similar to that used in Example I except that 26 g./l. of ferrous ammonium sulfate was used. Under the same operating conditions a coating of about 75 per cent chromium and the balance iron was formed.

The alloy plate had a hardness of about 900 to 1000 Knoop. When the plate was treated at 1110 F. for 12 hours, the hardness decreased only to about 800 Knoop, which is a value far superior to that obtained after similarly heating chromium plate from the customary chromic acid bath. A hardness of 550 Knoop was retained when the alloy was heated to 1470 F.

Example IV The bath of Example I was changed by the addition of 52 g./l. of ferrous ammonium sulfate instead of 13.5 g./l. Under the same operating conditions an alloy coating was formed comprising 45 per cent chromium and the balance iron, and having hardness-retaining properties comparable to the alloy plated in Example III.

Example V A plating bath was made up similar to that of Example I except that the ferrous ammonium sulfate was omitted. Using the same operating condition a chromium coating was formed on the cathode.

In carrying out most of the plating operations, glass tanks were used. However, it should be obvious that any material can be used which will not react with the electrolyte during the plating process. Koroseal-lined tanks have proved to be satisfactory.

It may be desirable to use a porous diaphragm to separate the anolyte from the catholyte in order to prevent excessive hexavalent chromium ions produced at the anode from contaminating the bath.

Heretofore, it has been necessary that no hexavalent chromium ions be present in a chromium plating bath. However, in the improved bath and process of this invention an appreciable concentration of hexavalent chromium can be tolerated without resulting in low electrical efficiency and poor quality plate.

If no diaphragm is used, an excessive amount of these hexavalent chromium ions can be indirectly reduced by the addition of an oxidizing agent such as hydrogen peroxide. The hexavalent chromium ions will be oxidized to form perchromate ions, which are unstable and revert to trivalent chromium ions. If too much hydrogen peroxide is added, the bath will stop plating, but this may be corrected by the addition of a small amount of per cent chromic acid solution.

To obtain best results in the plating, the bath should be maintained in a pH range of from about 0.2 to about 3.5, while the preferred range is from about 1.0 to about 2.8. It is possible to adjust the pH by adding a predetermined amount of ammonium hydroxide. If it is desired to add alkaline reagents to the bath after it has been prepared, special care must be taken to avoid the formation of precipitates which are relatively insoluble.

Sodium sulfate is an important constituent of the bath because of its beneficial effect on current efficiency and adherence of the alloy plate. The exact concentration of sodium sulfite, which should be used to effect satisfactory coatings is dependent upon the amounts of the other components present in the bath. If too small an amount of sodium sulfite is used, the coating produced will be flaky, non-continuous and non-adherent. The efficiency of the bath will also become very low after a period of use. The deposit from a bath having too high a percentage of sodium sulfite will be bright but streaked with a dark herringbone pattern. However, the excess of sodium sulfite can be compensated for by the addition of a small amount of ammonium persulfate. A satisfactory concentration is from about 0.001 to about 1 g./l., while a preferred amount of sodium sulfite would be from 0.005 to 0.2 g./l.

The magnesium sulfate facilitates the deposition of smooth deposits. If less than 10 gms. per liter are used, the electrodeposited alloy will be too rough for industrial applications. The concentration may be as high as 200 gms. per liter, but when it exceeds 150 gms. per liter there may be some modification of the physical properties of the coating. For optimum results the preferred range 18 from to 25 gms. per liter,

The amount of chromium ammonium sulfate, hereinafter referred to as chrome alum, used in the bath is dependent on the current density at which the bath is operated. Satisfactory results have been obtained with a concentration ranging from 200 gms. per liter to saturation. Good plates were obtained from a bath containing more than 1300 gms. per liter, but when the bath was cooled to 135 F., salt crystals precipitated. When the current density is between and 225 amps. per square foot, a chrome alum concentration of 300 gms. per liter results in a very good plate. For current density of from 300 to 400 amps. per square foot, a chrome alum concentration of 700 gms. per liter is preferred. In general, smaller amounts of chrome alum salt will result in better throwing power.

Ammonium sulfate is added to the bath to maintain good throwing power. It is particularly effective when the plating is done with a low current density and when low concentrations of chromium ammonium sulfate and ferrous ammonium sulfate are used. In general, satisfactory results are obtained using a concentration of from about 25 to about 175 g./l. A preferred range is from about 40 to about 100 g./l.

The plating bath may be operated over a wide range of temperatures provided the pH is adjusted properly. As has been previously stated, there is danger of precipitation of chrome alum from the more concentrated baths at temperatures below F. Alloys which have been electrodeposited at temperatures higher than 160 F. show a tendency to be pitted which is not desirable for a hard surface plate. The preferred temperature range lies between and F.

Satisfactory plating has been obtained using current densities of from 100 to 500 amps. per square foot. As heretofore shown, the selection of the proper current density is dependent upon the concentration of the chrome alum. Other factors affecting the choice of the proper current density include the size of the plating tank, the shape and contour of the parts, and the time required to produce a given thickness. For example, it has been found that the chromium alloy can be electrodeposited at approximately 0.002 inch per hour at 100 amps. per square foot, and 0.013 inch per hour at 500 amps. per square foot.

It is commonly known in commercial electroplating processes that as the metal or metals are electrodeposited out of the bath on to the cathode they must be replaced at the same rate. In the case of alloy electrodeposition, the metals must be replaced not only at the same rate but also substantially in the same ratio. When the plating baths of this invention are used, this replacement may be made when using either insoluble or soluble anodes, as well hereinafter be described.

When insoluble anodes are used they are commonly made from pure lead or a composition of 99% lead and 1% silver. These insoluble anodes may be placed inside porous cups or separated from the cathode-deposition bath by porous diaphragms. Such separation is advisable in order to prevent the chromium chromate which is formed by anodicoxidation at the anode from migrating back to the cathode. Excessive concentration of this chromate tends to decrease the efficiency of the electrodeposition.

If it is desired to omit the use of the porous cups or diaphragms, as has been previously shown, an oxidizing agent such as hydrogen peroxide should be added to the bath in an amount sufiicient to indirectly change the hexavalent chromium content of the bath back to the trivalent state.

It also is possible to eliminate the need for porous cups or diaphragms or the use of a reducing agent by using soluble anodes. These anodes may be constructed of magnesium metal or of a chromium alloy of substantially the same composition as that desired in the electrodeposited alloy.

Because of the wide permissible range of concentration of magnesium salt in the baths of this invention, soluble magnesium anodes can be used without the necessity of separating the anode and cathode reaction reagents. As the magnesium dissolves in the plating solution, it causes rfzio oxidation of the trivalent chromium to a hexavalent orm.

Since the apparent anode efiiciency for magnesium far exceeds the electrochemical equivalent, indicating some chemical dissolution at the anode during operation, an

aesam '5 accumulation: of magnesium ammoniumsulfate results. As this sulfate forms, thepH of" the bath steadily in creases. For short plating periods, this causes nordifli culty. However, for longer plating periods it may be necessary to periodically adjust the pH by adding asmall amount of sulfuric acid.

When it is desired to operatethe plating bath continuously and for long periods of: time, the most satisfactory results are obtained by using chromium-alloy anodes. 'When thistype of anode isused, a small amount of chloride must be added to facilitate the dissolution of the chromium alloy in the active state so that thechromium will be replenished in the trivalent (Cr form in the bath. If dissolution occurswhen the anode is passive," the chromium enters into the solution in the=hexava1en-t form (Cr+ and would necessarily have to be changed tothe, trivalent form by addition of hydrogen peroxide. As has been previously pointed out, the presence of an excessive amount of hexavalent chromium decreases the rate of deposition.

The chloride may be added in the form of suitable soluble chlorides such as ammonium chloride or chlorides of the coating metals with a concentration range of from about 10. to. 50 grams per liter. Since the chloride ion or anion is the important part of such additions,

the cation is not critical as. longas itdoes not: interfere with the proper action and control of the plating bath.

It can thus. be seen; that continuous. operation of the plating processis facilitated by using soluble anodes. The

construction is greatly simplified by the elimination of the porous cup or diaphragm to contain the analyte solution, and necessity for constant checking on the solution composition is greatly minimized or even eliminated. The only compensation that must be made is that which is necessary to replace the drag-out losses. This is done by the addition of chrome alum from a stock solution.

It is possible to operate the plating solution under such anode conditions that a slightly greater amount of metal is dissolved from the anode than is deposited at the cathode. When this is done, the bath can be adjusted merely by adding acid, since the excess metal dissolved from the anode will combine with the acid to replenish the salts lost by drag-out. Thus, it is not necessary to add chrome alum; and only magnesium salt, sodium sulfite and the chloride salt needto be added periodically in accordance. with analyses made for determining the proper control. conditions.

When insoluble anodesare used or when soluble magnesium anodes are used, the bath is not replenished in chromium or the alloy metal. As a result of continuous operation, the concentration of the chromium will diminish proportionately with. the time of electrodeoosition. These metals are constantly replaced byadditions of chromium and alloying metal compounds to the: bath. A very satisfactory method of replenishing the bath with the metals being deposited on the work is to make use of metal hydrates.

It is well known that chromic hydroxide is diflicultly and only slowly soluble in weak acid. Since the electroplating bath is a weak acid solution, considerable time delay would be involved by replenishing the chromium by adding chromic hydrates of the usual type. This difiiculty may be easily overcome by using a special form of chromium hydrate prepared in accordance with the method described in U. S. Patent No. 2,436,509. Chromic hydrate prepared by this method is easily handled, very easily filtered and washed free from impurities, and while wet or even after drying at low temperatures, dissolves very rapidly in the plating solution.

While the examples heretofore given have related only to chromium iron alloys, the electroplating bath of this invention may be readily adapted for depositing other chromium alloys. For example, molybdenum can be included in the alloy plating process for the purpose of electrodepositing ternary alloys of chromium, iron and molybdenum. The following example gives suitable bath compositions and plating conditions for electrodepositing such a plating:

Example VI Chromium ammonium sulfate [Cr2(SO4)3-(NH4)2SO4-24H2O] 300 to 700 g./l. Ferrous ammonium sulfate [FeSOi- (NH4)2SO4'6H2O] 13 to 50 g./l.

Molybdic acid' (anhydnidc) [M003] l1 toS-O 'gt/l; Magnesium sulfate [MgSO4'7I-I2O] p 10 to g./l. Sodium sulfite- INazSOs- 71-1203. 0.0.1 to 0.02 .g./l. Bath temperature 100-160F. Current density 100-4'50 amps.

per sq. ft. Anodes Insoluble (Pb or Pb The following example lists the bath, compositions and the plating conditions, for forming, an alloy coating of chromium, iron, and tungsten;

Example VII Chromium ammonium sulfate.

[Cr2SO4,)3' (NH4)2SQ4--24'H20] 300 to 700 g./l. Ferrous ammonium sulfate The preliminary preparation of the material prior to plating may include. any of the methods known. in the art, such as pickling, cleaning, etc.

It is also possible to electrodeposit chromium nickel alloys bysubstituting: nickel salt for the iron salt. Cobalt can also be used in the. bathfor electrodepositing cobalt chromium alloys. Zinc: may besubstituted for depositingzinc chromium alloys, and various ternary or even quaternary combinations of these metals can be electrodeposited in the form of adherent hard alloy plates.

For example, to electrodeposita cobalt-chromium alloy, cobalt ammonium, sulfate is a, substituted for the ferrous ammonium sulfate in the plating, solution. Similariy, for electrodepositing a chromium-iron-nickel altoy or a chromium-iron-cobalt alloy, either nickel or cobalt ammonium sulfate is added to the bath in addition tothe other constituentswhich have already been indicated. The ranges for the other metal sulfates are substantially the same as. those for the ferrous sulfates. Thus,- either cobalt and/or nickel-ammonium sulfate may be added to the bath in amounts in the range of from A to 60 grams per liter. The other conditions relating to current density, temperature, and pH remain substantially the same.

In practicing this invention the proper concentration of the desired metal ammonium sulfate may also be achieved by adding ammonium sulfate and a suitable metal salt separately in correct proportions. Any soluble salt may be used which does not introduce undesired ions in the bath.

It will, of course, be understood that various details of the process may be varied without departing from the principles of this invention. It is, therefore, not the purpose to limit the patent other than is necessitated by the scope of the appended claims.

What is claimed is:

1. The method of forming a coating containing chromium which comprises making an article to be coated the cathode in an aqueous solution, said solution consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./1. magnesium sulfate, from 25 to 175 g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, and the balance water, maintaining the temperature of the solution between and F., and passing an electric current of from 100 to 500 amp/sq. ft. therethrough between an anode and said cathode.

2. The method of forming a coating containing chromium which comprises making an article to be coated the cathode in an aqueous solution, said solution consisting essentially of from 300 to 700 g./l. chromium ammonium sulfate, from to g./1. magnesium sulfate, from to g./l. ammonium sulfate, from 0.005 to 0,2 g./l. sodium sulfite, and the balance water, maintaining the temperature of the solution be tween 140 and 150 F., and passing an electric current of from 100 to 500 amp/sq. ft. therethrough between an anode and said cathode.

3. The method of electrodepositing a coating containing chromium which comprises making an article to be coated the cathode in an aqueous solution, sald solution consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./l. magnesium sulfate, from 25 to 175 g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, the ions of at least one metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, and the balance water, maintaining the temperature of the solution between and 160 F., and passing an electric current of from 100 to 500 amp/sq. ft. therethrough between an anode and said cathode.

4. The method of electrodepositing a coating containing chromium which comprises making an article to be plated the cathode in an aqueous solution, said solution consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./l. magnesium sulfate, from 25 to 175 g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, the soluble salt of at least one metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, and the balance water, maintaining the temperature of the solution between 135 and 160 F., and passing an electric current of from 100 to 500 amp/sq. ft. therethrough between an anode and said cathode.

5. The method of electrodepositing a coating containing chromium which comprises making an article to be plated the cathode in an aqueous solution, said solution consisting essentially of from 300 to 700 g./l.

chromium ammomum sulfate, from 15 to 25 g./l. magnesium sulfate, from 40 to 100 g./l. ammonium sulfate, 0.005 to 0.2 g./l. sodium sulfite, the ions of at least one metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, and the balance water, maintaing the temperature of the solution between and F., and passing an electric current of from 100 to 500 amp/sq. ft. through said solution between an anode and said cathode.

6. The method of electrodepositing a coating 'containing chromium which comprises making an article to be plated the cathode in an aqueous solution, said solution consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./l. magnesium sulfate, from 25 to 175 g./l. am-

monium sulfate, from 0.001 to 1.0 g./l. sodium sulfite the ions of at least one additional metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, from 10 to 50 g./l. of at least one soluble chloride selected from the group consisting of the chlorides of magnesium, sodium, potassium, ammonium and said additional metal, and the balance water, maintaining the temperature of the solution between 135 and F., and passing an electric current of from 100 to 500 amp/sq. ft. between said cathode and a soluble alloy-anode of chromium and said additional metal.

7. An electrolyte for electrodepositing a coating containing chromium, said electrolyte consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./l. magnesium sulfate, from 25 to g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, and the balance water.

8. An electrolyte for electrodepositing a coating containing chromium, said electrolyte consisting essentially of from 200 g./l. to saturation of chromium ammonium sulfate, from 10 to 200 g./l. magnesium sulfate, from 25 to 175 g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, the ions of at least one metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, and the balance water.

9. An electrolyte for electrodepositing a coating containing chromium, said electrolyte consisting essentially of from 200 g./l. to saturation of chromium am monium sulfate, from 10 to 200 g./l. magnesium sulfate, fromv 25 to 175 g./l. ammonium sulfate, from 0.001 to 1.0 g./l. sodium sulfite, the ions of at least one additional metal selected from the group consisting of iron, nickel, zinc, tungsten, molybdenum and cobalt, from 10 to 50 g./l. of a soluble chloride selected from the group consisting of the chlorides of magnesium, sodium, potassium, ammonium, and said additional metal, and the balance Water.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Fuseya et al., Transactions Electrochemical Society, vol. 59 (1931), pp. 445-460.

Rogers et al., Transactions Electrochemical Society, vol. 64 (1933), pp. 299304. 

3. THE METHOD OF ELECTRODEPOSITING A COATING CONTAINING CHROMIUM WHICH COMPRISES MAKING AN ARTICLE TO BE COATED THE CATHODE IN A AQUEOUS SOLUTION, SAID SOLUTION CONSISTING ESSENTIALLY OF FROM 200 G./1. TO SATURATION OF CHROMIUM AMMONIUM SULFATE, FROM 10 TO 200 G./1. MAGNESIUM SULFATE, FROM 25 TO 175 G./1. AMMONIUM SULFATE, FROM 0.001 TO 1.0 G./1. SODIUM SULFITE, THE IONS OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSITING OF IRON, NICKEL, ZINC, TUNGSTEN, MOLYBDENUM AND COBALT, AND THE BALANCE WATER, MAINTAINING THE TEMPERATURE OF THE SOLUTION BETWEEN 135* AND 160* F., AND PASSING AN ELECTRIC CURRENT OF FROM 100 TO 500 AMP./SQ. FT. THERETHROUGH BETWEEN AN ANODE AND SAID CATHODE. 